A battery is a device that produces electrical energy through electrochemical oxidation and reduction reactions, and has a wide range of various applications. For example, application of a battery is gradually expanding to a power source of handheld portable devices such as a mobile phone, a laptop computer, a digital camera, a video camera, and an electric tool; various types of electric-powered devices such as an electric bike, an electric motorcycle, an electric vehicle, a hybrid vehicle, an electric boat, and an electric aircraft; an energy storage system used to store energy produced through new renewable energy or excess energy in an electricity-generating plant; and an uninterruptible power supplier for stable power supply to various information and communication devices including a server computer and a base station for wireless communication.
A battery includes three basic elements; one is an anode including a material which oxides while emitting electrons during discharging, another is a cathode including a material which reduces while accepting electrons during discharging, and the other is an electrolyte which allows ions to move between the anode and the cathode.
A battery may be classified into a primary battery that cannot be reused after discharged, and a secondary battery that can be charged and discharged repeatedly due to at least partially reversible electrochemical reactions.
As a secondary battery, a lead-acid battery, a nickel-cadmium battery, a nickel-zinc battery, a nickel-iron battery, a silver-oxide battery, a nickel metal hydride battery, a zinc-manganese dioxide battery, a zinc-bromine battery, a metal-air battery, and a lithium secondary battery are known. Among them, a lithium secondary battery has a higher energy density, a higher battery voltage, and a longer lifespan than the other secondary batteries, and for these reasons, is attracting the greatest attention in commercial aspects.
A lithium secondary battery has a characteristic that intercalation and de-intercalation reactions of lithium ions occur at a cathode and an anode. That is, during discharging, lithium ions de-intercalated from an anode material included in an anode moves to a cathode through an electrolyte and are intercalated into a cathode material included in the cathode, and vice versa during charging.
In the lithium secondary battery, because a material used as a cathode material significantly affects performance of the secondary battery, various attempts have been made to provide a cathode material having a high energy capacity while maintaining stability at high temperature as well as having low manufacturing costs. However, use of only one cathode material has a limitation in satisfying all the industrial performance standards.
Recently with the growing concerns on exhaustion of fossil fuels and air pollution, there is a drastic increase in demand for eco-friendly energy. In this context, commercialization of an electric drive vehicle such as an electric vehicle or a hybrid vehicle that is powered and runs by electrical energy supplied from a secondary battery is being accelerated by developed countries.
When an electric drive vehicle runs, a state of charge (SOC) of a secondary battery is a parameter needed to estimate a residual driving distance of the electric drive vehicle, and to control the start and end of the charge or discharge of the secondary battery.
A SOC of a secondary battery may be estimated correctly by measuring an open-circuit voltage of the secondary battery. This is because a SOC of a secondary battery has a one-to-one relationship with an open-circuit voltage of the secondary battery. However, it is impossible to measure an open-circuit voltage of a secondary battery during charging or discharging of the secondary battery.
Accordingly, conventionally, complex mathematical models or an experimentally-made lookup table capable of mapping an open-circuit voltage with temperature and voltage of the secondary battery were used to estimate an open-circuit voltage of a secondary battery.
However, the former method has a disadvantage of requiring a complicated calculation, and the latter method has a drawback in that accuracy reduces when applied during charging or discharging of a secondary battery under a dynamic condition. Particularly, in the case of an electric vehicle or a hybrid vehicle, when a driver works an accelerator pedal, a secondary battery is discharged at rapidly changing discharge rates (C-rate), and when the driver works a brake pedal, the secondary battery performs regeneration charging, and this process repeats. Therefore, there is a need for a new approach to estimate a SOC of a secondary battery conveniently and correctly in a dynamic usage environment of the secondary battery.